The invention relates generally to hard disk drives and more precisely to a method of self-testing magneto-resistive heads for instability after the heads have been incorporated in a hard disk drive.
In hard disk drives, data is written to and read from magnetic recording media, herein called disks, utilizing magneto-resistive transducers commonly referred to as MR heads. Typically, one or more disks are rotatably mounted on a spindle. An MR head, mounted on an actuator arm, is positioned in close proximity to the disk surface to write data to or read data from the disk surface.
During operation of the disk drive, the actuator arm moves the MR head to a desired radial position on the surface of the rotating disk where the MR head electromagnetically writes data to the disk and senses magnetic field signal changes to read data from the disk. Usually, the MR head is integrally mounted in a carrier or support referred to as a xe2x80x9csliderxe2x80x9d. A slider generally serves to mechanically support the MR head and any electrical connections between the MR head and the disk drive. The slider is aerodynamically shaped, which allows it to fly over and maintain a uniform distance from the surface of the rotating disk. In typical disk drives, contact between the MR head and disk is extremely undesirable.
Typically, a slider is formed with two or more parallel rails with a recessed area between the rails and with each rail having a ramp at one end.
The surface of each rail that glides over the disk surface during operation is known as the air-bearing surface.
An MR head typically includes an MR read element that reads recorded data, and an inductive electromagnetic element that writes the data. Both the MR read element and inductive electromagnetic element terminate at the air-bearing surface.
To manufacture MR heads, rows of MR heads are deposited simultaneously on the surface of a semiconductor wafer using known semiconductor process methods. After deposition of the MR heads is complete, bars are sliced from the wafer, each bar comprising a row of units which can be further processed into sliders having one or more MR heads on their end faces. Each bar is bonded to a fixture where the bar is further processed and then diced, or separated, into individual sliders, each slider having at least one MR head terminating on the slider""s air bearing surface.
To achieve maximum efficiency from MR heads, dimensions near the pole tips must be especially carefully controlled. Two of these dimensions are the xe2x80x9cthroat heightxe2x80x9d for thin film inductive electromagnetic elements and the xe2x80x9cMR read element heightxe2x80x9d in the case of MR read elements, which must be maintained within a certain limited tolerance for generating the maximum signal from a given MR head element. During row bar processing, it is critical to grind or lap the bar to the desired thickness to achieve the desired throat height and MR element height.
Conventional lapping processes utilize either oscillatory or rotary motion of the work piece (i.e., the row bar) across either a rotating or oscillating lapping plate to move the work piece randomly over the lapping plate thereby randomizing plate imperfections across the MR head surface caused by the lapping process. During the lapping process, the motion of abrasive grit carried on the surface of the lapping plate is typically transverse to the MR head elements exposed at the slider air-bearing surface. In MR heads, the electrically active components as well as other magnetic components exposed at the air-bearing surface are made of relatively soft materials. During the lapping process, these soft conductive components can scratch and smear over insulator layers in the read gap creating electrical shorts or effectively narrowing the read gap so that electrical leakage can cause unstable performance of the MR head.
It is also known that for an MR head to function effectively, it must be subjected to a transverse bias field to linearize its response to magnetic field signal changes. Various transverse biasing techniques are known including current shunt, xe2x80x9cbarber polexe2x80x9d and soft adjacent film biasing. The transverse bias field is applied normal to the plane of the magnetic media and parallel to the surface of the MR head.
It is also known that an MR head may be utilized with a longitudinal bias field extending parallel to the surface of the magnetic media and parallel to the major axis of the MR head. Stabilization of MR heads by means of a longitudinal bias field is necessary to suppress Barkhausen noise if the MR heads are used in high track density disk drives. Barkhausen noise results from unstable magnetic properties such as multi-domain states (or domain walls) within the MR read element portion of the MR head which may appear or move following a magnetic disturbance from the associated inductive electromagnetic element portion of the MR head or other external magnetic field source.
With respect to the last problem, MR read elements are commonly stabilized with ferromagnetic materials such as ferromanganese or a permanent magnet layer comprising cobalt platinum, cobalt platinum tantalum or cobalt platinum chromium in order to obtain a single magnetic domain state throughout the MR element. Stabilizing the MR read element with a ferromagnetic layer is commonly referred to as pinning the off-track boundaries of the MR read element. However, the effectiveness of such boundary bias methods diminishes in the center of the MR read element due to the fact that magnetic flux rapidly leaks out of the MR read element as the distance to the boundary of the MR read element increases. This undesired flux leakage is one common cause of multi-domain states and associated Barkhausen noise on reading data, resulting in MR head instability problems.
During the deposition of the MR read element layers, pinholes, defects and contaminant particles can be formed in the insulation layers, which results in dielectric breakdown. The electrical conductivity of such layers may suddenly increase in sporadic and unexpected ways to change the apparent resistance of the MR read element. These unexpected changes in resistance of the MR read element contribute to MR head instability. (See B. R. Baker, xe2x80x9cELECTROSTATIC POPPING IN AMR AND GMR HEADSxe2x80x9d, IEEE Trans. Mag., vol. 35, pp. 2583-2885, 1999.)
MR head instability caused by Barkhausen noise and dielectric breakdown results in sudden voltage baseline jumps in the read back waveform. This can adversely affect the data error rate in the read channel and more seriously affect the servo positioning feedback system of the head positioner incorporated with the disk drive. In other words, MR head instability seriously affects the disk drive""s ability to position the MR head over the recorded data on the disk surface correctly, and thus making reliable reading of the recorded data impossible.
Another cause of voltage baseline jumps in the read back signal occurs from the impact of the MR head with a small bump projecting from the disk surface. These bumps are known as thermal asperities. When an MR head strikes a thermal asperity there is usually a rapid transfer of energy and associated temperature rise near the impact area. If the MR head""s temperature changes abruptly as a result of striking a thermal asperity, then its resistance also rises rapidly causing the read signal to exhibit a voltage baseline jump with a decaying tail. These voltage baseline jumps resemble those caused by Barkhausen noise or dielectric breakdown, but the thermal asperities give voltage baseline jumps of only one polarity, depending on the connections between the MR head and preamplifier.
Disk drives containing disks with a small number of thermal asperities are usable because areas on the disks containing the thermal asperities can be avoided during use of the disk drive. Disk drives containing MR heads with dielectric breakdown or Barkhausen noise may not be usable because there is no practical way to avoid using the MR heads contained in the disk drive. Voltage baseline jumps generated by Barkhausen noise, dielectric breakdown or thermal asperities are very similar, making it difficult to distinguish between them and thus making it difficult to determine if a disk drive exhibiting the voltage baseline jumps can be reliably used.
Thus, a hitherto unsolved need has remained for a method of testing MR heads to identifying and distinguish between MR heads exhibiting voltage baseline jumps resulting from intermittent instability problems associated with Barkhausen noise, dielectric breakdown or thermal asperities.
An objective of the present invention is to provide a self-test method for an MR head incorporated within a hard disk drive (xe2x80x9cHDDxe2x80x9d). The self-test method is used to determine if voltage baseline jumps are present in the read back signal sensed by the MR head and if such voltage baseline jumps are caused by MR head magnetic instability (Barkhausen noise), dielectric breakdown, or contact with the disk at a thermal asperity. Because this is a self-test method carried out by the disk drive""s electronics and firmware, it can be done during initialization and burn-in at the factory.
The MR head self-testing method, according to a first embodiment of the present invention, comprises spinning up the disk drive incorporating the MR head, enabling the MR head to fly over the disk, in close proximity thereto. A suspension assembly, which supports the MR head over the rotating storage disk, is controlled to move the MR head over a track of the disk that can be used for testing (xe2x80x9ctest trackxe2x80x9d). Any previously written data in the data fields is erased. The MR head is controlled to sense a read signal from the data fields on the test track. The read signal is provided to a first filter, where the first filter conditions the read signal by removing high frequency noise from the read signal. The filtered read signal is provided to a second filter, where the second filter further conditions the filtered read signal using preprogrammed filter coefficients contained in the second filter. The read error signal provides exaggeration to any voltage baseline jumps originally present in the filtered read signal. The read error signal is applied to a digital comparator and counter circuit. The digital comparator and counter circuit detects and counts each positive and each negative voltage baseline jump in the read error signal according to preprogrammed positive and negative voltage threshold values contained in the digital comparator and counter circuit. The detected values of the counter, which represents the detected positive and negative voltage baseline jumps, is stored in an error diagnostic register for analysis. If the error diagnostic register contains values indicative of voltage baseline jumps of a single polarity, the voltage baseline jumps may be caused by thermal asperities. If the error diagnostic register contains values indicative of both positive and negative voltage baseline jumps, then at least some of the voltage baseline jumps may be caused by Barkhausen noise or dielectric breakdown.
This self-test circuit and method enables the use of disk drives with thermal asperities while disqualifying disk drives with Barkhausen noise or dielectric breakdowns in the MR head.
These and other objects, advantages, aspects and features of the present invention will be more fully understood and appreciated upon consideration of the following detailed description of a preferred embodiment, presented in conjunction with the accompanying drawings.